In order to meet the convenience of users or improve reliability, it is desirable that while apparatus are in operation, maintenance or replacement can be performed. Accordingly, electronic devices which can be plugged or unplugged with power turned on (hot swap) have traditionally been used widely. When such an electronic device is newly plugged with power turned on, no electric charge is stored by an input capacitor which stabilizes an electric current in a circuit, and charging current flows to the input capacitor. This charging current is referred to as inrush current and is excessive surge current. This surge current causes a protective fuse to blow, causes a connector to melt and adhere, or causes a momentary drop in supplied voltage. This leads to damage to an electronic device or a temporary stoppage of the operation. In order to prevent this inrush current, an inrush current control circuit, for example, is used. Usually a method utilizing control of the ON-state resistance of an FET (Field Effect Transistor) is adopted.
Furthermore, generally two or more power supplies are used. Even if a failure occurs in one power supply, an apparatus can be operated continuously. These two or more power supplies are integrated into one system by the use of back flow preventing diodes and power is supplied to a circuit.
The following power supply apparatus is proposed. A diode is connected in a forward direction to direct-current input and an input voltage detection circuit is placed between the diode and the direct-current input. A loss of a switching element at a low input voltage needs to be controlled. When input voltage is detected, base current in a drive circuit of the switching element is increased, collector current in the drive circuit is increased, and a resonance current for a tank circuit is supplied stably. By doing so, spike noise is controlled and the loss is controlled (see, for example, Japanese Laid-open Patent Publication No. 05-219756).
There are cases where power is applied and disconnected repeatedly in a short period of time. In a hot swap of a circuit board, for example, connector chattering occurs or unplugging and plugging are repeated in a short period of time. As a result, excessive inrush current may flow in a circuit. Furthermore, if a connector is electrically connected and disconnected many times successively, power is repeatedly applied and disconnected many times in a short period of time. As a result, excessive inrush current flows many times in a circuit because of electric charges stored by a capacitor. This excessive inrush current causes a fuse to blow, causes a connector to melt and adhere, or causes a drop in power supply voltage. This leads to damage to a circuit board or a temporary stoppage of the operation.